patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.
diff --git a/src/testdir/test_signals.vim b/src/testdir/test_signals.vim
index 44732df..cc1c3fb 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -1,8 +1,7 @@
 " Test signal handling.
 
-if !has('unix')
-  throw 'Skipped: not on Unix'
-endif
+source check.vim
+CheckUnix
 
 source shared.vim
 
@@ -14,8 +13,9 @@
 
 " Test signal WINCH (window resize signal)
 func Test_signal_WINCH()
-  if has('gui_running') || !HasSignal('WINCH')
-    return
+  CheckNotGui
+  if !HasSignal('WINCH')
+    throw 'Skipped: WINCH signal not supported'
   endif
 
   " We do not actually want to change the size of the terminal.